The present invention relates to fuels for internal combustion engines. More particularly, it relates to the production of a fuel for compression ignition engines. Within this field, the invention relates to a process for transforming a gas oil cut to produce a dearomatised and desulphurised fuel with a high cetane number.
Gas oil cuts, whether straight run from a crude petroleum or from a catalytic cracking process, currently still contain non negligible quantities of aromatic compounds, nitrogen-containing compounds and sulphur-containing compounds. The current legislation of the majority of industrialised countries dictates that engine fuel must contain less than 500 parts per million (ppm) of sulphur. Some countries have no current regulations which impose a maximum aromatics and nitrogen content. However, several countries or states, like Sweden and California, are known to be planning to limit the aromatics content to less than 20% by volume, or even to less than 10% by volume, and some experts believe that this limit could be 5% by volume. In Sweden in particular, certain classes of diesel fuel must already satisfy very severe specifications. Thus in that country, class II diesel fuel cannot contain more than 50 ppm of sulphur and more than 10% by volume of aromatic compounds, and that of class I cannot contain more than 10 ppm of sulphur and 5% by volume of aromatic compounds. Class III fuel in Sweden must currently contain less than 500 ppm of sulphur and less than 25% by volume of aromatic compounds. Similar limits also apply for the sale of that type of fuel in California.
During this time, motorists in several countries have pressed for legislation which will oblige petrol producers to produce and sell a fuel with a minimum cetane number. Current French legislation requires a minimum cetane number of 49, but in the near future this minimum number could be at least 50 (as is already the case for class I fuel in Sweden), and probably at least 55; most probably it will be between 55 and 70.
A number of specialists are of the serious view that in the future the nitrogen content will be regulated, to less than 200 ppm, for example, or even less than 100 ppm. A low nitrogen content would improve the stability of the products, which would be welcomed by both the product vendor and the producer.
A reliable and efficient process thus needs to be developed, which process can produce a product with improved characteristics regarding the cetane number and the aromatics, sulphur and nitrogen content, from conventional straight run gas oil cuts or those from catalytic cracking (LCO cut) or from a different conversion process (coking, visbreaking, hydroconversion of residues, etc.). It is particularly important, and this is one of the advantages of the process of the present invention, to produce a minimum of gaseous hydrocarbon compounds and to be able to produce an effluent which is directly and integrally saleable as a very high quality fuel cut. Further, the process of the present invention can be carried out and produced over a long period of time without the need for regeneration of the catalysts used, which have the advantage of being very stable over time.
In its broadest scope, the present invention thus concerns a process for transforming a gas oil cut to produce a dearomatised and desulphurised fuel with a high cetane number in at least two successive steps. It also concerns the fuel obtained by this process.
More precisely, the present invention concerns a process for transforming a gas oil cut into a dearomatised and desulphurised fuel with a high cetane number, comprising the following steps:
a) at least one first step for deep desulphurization and deep denitrogenation in which the gas oil cut and hydrogen are passed over a catalyst comprising a mineral support, at least one metal or metal compound from group VIB of the periodic table in a quantity, expressed as the weight of metal with respect to the weight of finished catalyst, of about 0.5% to 40%, at least one metal or metal compound from group VIII of the periodic table in a quantity, expressed as the weight of metal with respect to the weight of finished catalyst, of about 0.1% to 30% and phosphorous or at least one phosphorous compound in a quantity, expressed as the weight of phosphorous pentoxide with respect to the weight of the support, of about 0.001% to 20%; and
b) at least one subsequent second step for dearomatisation in which at least a portion, preferably all, of the product from the first step which has been at least partially and preferably completely desulphurised and denitrogenated is passed with hydrogen over a catalyst comprising, on a mineral support, at least one noble metal or noble metal compound from group VIII in a quantity, expressed as the weight of metal with respect to the weight of finished catalyst, of about 0.01% to 20%, and preferably at least one halogen.
Advantageously, in accordance with the process, hydrogen is introduced at each first and second step, and may be recycled to the first and second steps, independently of each other, meaning that the gases from the two steps are not handled together.
The effluent from the first step preferably undergoes steam stripping to separate at least part of the gas phase, which may be treated and optionally recycled at least in part to that step. At least a portion of the product from the stripping step undergoes the second step of the process of the invention.
The effluent from the final step is preferably steam stripped, is advantageously passes into a coalescer and is optionally dried.
In a preferred implementation of the invention, the operating conditions of steps a) and b) are selected as functions of the characteristics of the feed which may be a straight run gas oil cut, a gas oil from catalytic cracking or a gas oil from coking or visbreaking of residues, or a mixture of two or more of these cuts so as to obtain a product containing less than 100 ppm of sulphur and less than 200 ppm, preferably 50 ppm, of nitrogen and the conditions of step b) are selected so that the product obtained contains less than 10% by volume of aromatic compounds. These conditions may be rendered more severe so as to obtain, after the second step, a fuel containing less than 5% by volume of aromatic compounds, less than 50 ppm or even less than 10 ppm of sulphur, less than 20 ppm, or even less than 10 ppm of nitrogen, and with a cetane number of at least 50 or even at least 55, generally in the range 55 to 60.
To obtain such results, the conditions of step a) include a temperature of about 300xc2x0 C. to about 450xc2x0 C., a total pressure of about 2 MPa to about 20 MPa and an overall hourly space velocity of the liquid feed of about 0.1 to about 10, preferably 0.1 to 4, and those in step b) include a temperature of about 200xc2x0 C. to about 400xc2x0 C., a total pressure of about 2 MPa to about 20 MPa and an overall hourly space velocity of about 0.5 to about 10.
When a relatively low pressure range is desired while still producing excellent results, a first step a1) can be carried out under conditions which can reduce the sulphur content of the product to about 500 to 800 ppm, then this product can be sent to a subsequent step a2) the conditions of which are selected to bring the sulphur content to a value which is below about 100 ppm, preferably below about 50 ppm, and the product from this step a2) is then sent to step b). In this implementation, the conditions of step a2) are identical or, as is preferable, milder than when a single step a) is used with a given feed, since the product sent to this step a2) already has a greatly reduced sulphur content. In this implementation, the catalyst in step a1) can be a conventional prior art catalyst such as that described in the text of our French patent applications FR-A-2 197 966 and FR-A-2 538 813 and that of step a2) is that described above for step a). The scope of the invention includes using the same catalyst in steps a1) and a2).
In these steps a), a1) and a2), the catalyst support can be selected from the group formed by alumina, silica, silica-aluminas, zeolites, titanium oxide, magnesia, zirconia, clays and mixtures of at least two of these mineral compounds. Alumina is most frequently used.
In a preferred implementation of the invention, the catalyst in these steps a), a1), a2) will comprise, deposited on the support, at least one metal or metal compound, advantageously selected from the group formed by molybdenum and tungsten and at least one metal or metal compound advantageously selected from the group formed by nickel, cobalt and iron. The catalyst most frequently comprises molybdenum or a molybdenum compound and at least one metal or metal compound selected from the group formed by nickel and cobalt.
In a particular and preferred implementation of the invention, the catalyst in steps a), a1) and a2) comprises boron or at least one boron compound, preferably in a quantity of 10% or less, expressed as the weight of boron trioxide with respect to the weight of the support, preferably deposited on the support.
The quantity of group VIB metal or metal compound (preferably Mo), expressed as the weight of metal with respect to the weight of finished catalyst, is preferably about 2% to 30%, more preferably about 5% to 25%, and that of the group VIII metal or metal compound (preferably Ni or Co) is preferably about 0.5% to 15%, more preferably about 1% to 10%.
A catalyst containing Ni, Mo, and P is preferably used, the proportions of these elements having been defined above, or more preferably Ni, Mo, P and B.
A particularly advantageous catalyst is that described in European patent EP-A-0 297 949, the disclosure of which is hereby incorporated.
This catalyst comprises: a) a support comprising a porous mineral matrix, boron or a boron compound and phosphorous or a phosphorous compound, and b) at least one metal or metal compound from group VIB of the periodic table and at least one metal or metal compound from group VIII of the periodic table, in which the sum of the quantities of boron and phosphorous, respectively expressed as the weight of boron trioxide (B2O3) and phosphorous pentoxide (P2O5) with respect to the weight of the support, is about 5% to 15%, preferably about 8% to 12% and advantageously about 8% to 11.5%, the atomic ratio of boron to phosphorous (B/P) being about 1.05:1 to 2:1, preferably about 1.1:1 to 1.8:1. Advantageously, at least 40% and preferably at least 50% of the total pore volume of the finished catalyst is contained in pores with an average diameter of more than 13 nanometers.
The catalyst preferably has a total pore volume in the range 0.38 to 0.51 cm3 xgxe2x88x921.
The quantity of group VIB metals or metal compounds contained in the catalyst is normally such that the atomic ratio of phosphorous to the group VIB metal or metals (P/VIB) is about 0.5:1 to 1.5:1, preferably about 0.7:1 to 0.9:1.
The respective quantities of group VIB metal or metals and group VIII metal or metals contained in the catalyst are normally such that the atomic ratio of group VIII metal or metals to group VIB metal or metals (VIII/VIB) is about 0.3:1 to 0.7:1, preferably about 0.3:1 to about 0.45:1.
The quantity by weight of the metals contained in the finished catalyst, expressed as the weight of metal with respect to the weight of the finished catalyst, is normally about 2% to 30%, preferably about 5% to 25%, for the group VIB metal or metals, and about 0.1% to about 15%, more particularly about 0.1% to 5%, for the group VIII metal or metals, and preferably about 0.15% to 3% in the case of noble group VIII metals (Pt, Pd, Ru, Rh, Os, Ir) and about 0.5% to 15%, preferably about 1% to 10%, in the case of non noble group VIII metals (Fe, Co, Ni).
In step b), the mineral support can be selected from the group formed by alumina, silica, silica-aluminas, zeolites, titanium oxide, magnesia, boron oxide, zirconia, clays and mixtures of at least two of these mineral compounds. The support preferably comprises at least one halogen selected from the group formed by chlorine, fluorine, iodine and bromine, preferably chlorine and fluorine. In an advantageous embodiment, the support comprises chlorine and fluorine. The quantity of halogen is normally about 0.5% to about 15% by weight with respect to the weight of the support. The support is normally alumina. The halogen is normally introduced into the support by the corresponding acid halide and the noble metal, preferably platinum or palladium is introduced, for example, from aqueous solutions of their salts or compounds such as hexachloroplatinic acid in the case of platinum.
The quantity of noble metal (preferably Pt or Pd) in the catalyst in step b) is preferably about 0.01% to 10%, usually about 0.01% to 5%, and generally about 0.03% to 3%, expressed as the weight of metal with respect to the weight of finished catalyst.
A particularly advantageous catalyst is described in FR-A-2 240 905, the disclosure of which is hereby incorporated. It comprises a noble metal, alumina, and a halogen, and is prepared by mixing the aluminous support with a noble metal compound and a reducing agent with formula AlXyR3-y where y is 1, 3/2 or 2, X is a halogen and R is a monovalent hydrocarbon radical.
A further highly suitable catalyst is that described in U.S. Pat. No. 4,225,461. It comprises a noble metal and a halogen and is prepared in a particular manner.
The following examples illustrate the invention without limiting its scope.